Communication systems often need to modulate a baseband signal to radio frequency (RF) for transmission, or demodulate an RF signal to baseband for reception. Usually, the initial signal is mixed with a signal at a predetermined frequency, to produce an intermediate frequency (IF) signal. The IF signal is commonly mixed again to produce a signal at a desired frequency band. In a typical modern communication system, a programmable frequency IC, or frequency synthesizer, is often used to provide the predetermined frequency signal.
The following example illustrates how a frequency synthesizer is used to provide the local oscillator (LO) frequency in a receiver. FIG. 1 is a block diagram illustrating a superheterodyne receiver similar to one that is used in a cellular telephone. The received RF signal is first amplified by low noise amplifier (LNA) 100. Mixer 105 mixes the amplified RF signal with another signal generated by RF LO 125, to produce an IF signal. The resulting IF signal is filtered by filter 110, and then amplified by variable gain amplifier (VGA) 115. Mixer 120 mixes the resulting signal with a fixed frequency signal generated by IF LO 130 to produce the signal in the desired baseband.
IF LO 130 is commonly implemented using a frequency synthesizer. The synthesizer has registers that are programmed with parameters needed to generate the desired IF frequency. FIG. 2 illustrates how a synthesizer is configured in a conventional implementation. External microcontroller 200 loads configuration parameters into synthesizer's registers 205 to configure phase locked loop and voltage controlled oscillator (PLL-VCO) 210 to generate the desired frequency. Block 215 is the synthesizer containing registers 205 and PLL-VCO 210; the rest of the synthesizer components are not shown. In existing implementations, the microcontroller writes multiple register values to the synthesizer's registers via a bus, typically a serial bus, thus slowing down the initialization process.
Another disadvantage of current implementations is that whenever the synthesizer is powered off, the configuration parameters in the registers are lost and must be reloaded the next time the synthesizer is powered on. Communication devices that employ transceivers, such as GSM phones, are often designed to turn the transceiver off between bursts to save power. In the case of a cellular phone, the transceiver's synthesizer is turned off and on many times during a phone call. The synthesizer transitions from an inactive state to an active state frequently and its power is turned off and on repeatedly. As a result, the registers must be reloaded during every power cycle, causing long initialization times as well as unnecessary power consumption. There is a need to reduce the synthesizer initialization time and power consumption caused by repeated register reloads.